This invention relates to a coating method in which a coating solution is applied to a continuously moving web to form a coated film thereon and an apparatus for practicing the method, and more particularly to a coating method in which an excessive amount of coating solution applied to the web is scraped off to form a coated film having a desired thickness, to thereby improve the so-called "metering system", and an apparatus for practicing the method.
The term "web" as used herein is intended to refer to a relatively long flexible belt-shaped support of a plastic film made of cellulose triacetate, polyamide, polyimide, polycarbonate, polyethylene terephthalate or polyvinyl chloride; or which is made of paper or synthetic paper or of a metal foil of aluminum, copper or the like; or which is made of a sheet of glass, ceramic or the like.
The term "coating solution" as used herein is intended to mean various coating solutions which are prepared in various solution compositions according to their objects of use. Examples of coating solutions are those for forming a photographic emulsion layer, a base layer, a protective layer, a back layer or the like in a photographic sensitized material; or those for forming a magnetic layer, a base layer, a lubricant layer, a protective layer, a back layer or the line in a magnetic recording medium, or those for forming an adhesive layer, a coloring layer, a rust proofing layer, etc. These coating solutions contain water soluble binders or organic binders.
A conventional coating means such as a dip, reverse roll, gravure roll, extrusion hopper or slide hopper is used to excessively apply the above-described coating solution to a web which is continuously run. Thereafter, a metering means such as an air knife, blade or wire bar is abutted against the layer formed on the web to scrape off an excessive amount to leave a desired thickness of coating solution.
However, the aforementioned metering means are disadvantageous in the following points:
1. In the case of the air knife, it requires substantial time and effort to uniformly set the air pressure distribution in the widthwise direction of the web and to maintain the air pressure distribution. The air knife suffers from problems in that the coating solution scraped off by the blowing air is scattered or forms bubbles, thus degrading the surface quality of the coated film, and lowering the recovery rate of the removed coating solution.
2. The blade's sharp edge abuts the coated film to remove an excessive amount of coating solution threfrom. Accordingly, the edge is greatly worn depending on the material of the web and the composition of the coating solution. Thus, the thickness and surface quality of the coated film cannot be shiftably controlled without frequently performing maintenance of an inspection of the edge.
3. In general, a wire bar which is formed as follows has been extensively employed: A metal wire is wound on a rod member to form a coil whose turns are close to one another. The outside diameter of the coil formed on the rod member is usually set between 10 and 20 mm.
The wire bar will be described in more detail with reference to FIG. 1. In this figure, the aforementioned coating means 1 is used to apply a coating solution L, the amount of which is usually several times the final amount of coating solution, to a web W which runs continuously in the direction of the arrow A. The aforementioned wire bar 2 is set across the web W and downstream of the coating means 1 and is pushed against the film LL of coating solution L coated on the web, so that an excessive amount EL of coating solution is scraped off the coated film LL by the outer wall of the wire member of the wire bar. Thus, only the coated film passed through the gap between the surface of the web W and the outer wall of the wire member wound on the wire rod becomes the final coated film, which is thereafter dried.
The above-described wire bar 2 is a useful metering technique because it is very simple in construction, maintenance and handling, and can provide a coated film of stable surface quality.
However, with respect to the scraping off of an excessive amount of coating solution and the provision of a stable surface quality, the use of the wire bar 2 is limited in the case where a coating solution L of high viscosity is applied at high speed to the web to form a thin coated film thereon.